1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a throat formed between adjacent stator vanes.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
In a gas turbine engine, the turbine converts the energy of the passing hot gas flow into mechanical energy to drive the rotor shaft. In an aero engine, the turbine provides a majority of the mechanical power to the fan. In an industrial gas turbine (IGT) engine, the majority of power delivered to the rotor shaft is used to drive an electric generator for electrical power production. In either case, the efficiency of the engine is directly related to the efficiency of the turbine.
One method of improving the efficiency of the turbine is to place a row of stator or guide vanes directly upstream from a stage of rotor blades in order to direct the hot gas flow into the rotor blades at the most opportune angle to produce the greatest reaction. The nozzle guide vanes have two principal functions. First, they must convert part of the gas heat and pressure energy into dynamic or kinetic energy, so that the gas will strike the turbine blades with some degree of force. Second, the nozzle vanes must turn this gas flow so that it will impinge on the turbine blades in the proper direction; that is, the gasses must impact on the turbine blade plane of the rotor. The nozzle does its first job by using the Bernoulli theorem. As through any nozzle, when the flow area is restricted, the gas will accelerate and a large portion of the static pressure in the gas is turned into dynamic pressure. The degree to which this effect will occur depends upon the relationship between the nozzle guide vane inlet and exit areas, which, in turn, is closely related to the type of turbine blade used.
Adjacent nozzles form a throat between the suction side wall of one vane and the pressure side wall of the adjacent vane. Making the nozzle area too small will restrict the airfoil through the engine, raise compressor discharge pressure, and bring the compressor closer to stall. Nozzle area is especially critical during acceleration, when the nozzle will have a tendency to choke (gas flowing at the speed of sound). Small exit areas also cause slower accelerations because the compressor will have to work against an increased back pressure. Increasing the nozzle diaphragm area will result in faster engine acceleration, less tendency to stall, but higher specific fuel consumption.
Therefore, a precise control of the throat size of a stator vane set is important in the efficient operation of the turbine. Important dimensions for turbine nozzles are shown in FIG. 1 and include the thickness of the trailing edge A of the stator vanes and the distance from side walls B of adjacent vanes.
Another method of improving the efficiency of the engine is to coat the turbine airfoils with a thermal barrier coating (or, TBC) in order to allow for exposure to higher gas flow temperatures or reduced cooling air allotment and associated losses. In one prior art stator vane set, the nozzles are coated with a TBC around the entire circumference of the airfoil as seen in FIG. 2. Adding a TBC of thickness T to the airfoils will reduce the airfoil throat at the exit end by 2T and increases the trailing edge diameter of the vane by 2T. Recent advances in coating technology have resulted in a TBC thickness increased to levels as great as approximately 1.0 mm thick. This high thickness of the TBC has a significant impact on the critical aerodynamic dimensions of the nozzles as represented in FIG. 1.
FIG. 3 shows a prior art airfoil with a constant taper of the airfoil trailing edge contour C and a TBC applied in which the TBC tapers off from normal thickness to a zero thickness in which the metallic material of the airfoil at the trailing edge is exposed. This produces an airfoil with a surface contour that will be aerodynamically undesirable.
The prior art aerodynamic design accounts for the effect of TBC thickness when setting the airfoil throat dimension B, but tends to accept the increased thickness in dimension A. limitations of the prior art design practice are spallation of TBC results in a significant variation of the throat area over the life of the part, and increased aerodynamic losses associated with high trailing edge thickness.